This invention is generally directed to the function of providing an RF bypass function for tower-mounted low noise amplifiers (LNAs).
The present invention is directed to the operation of amplifiers, such as low noise amplifiers in a tower-mounted active antenna system, such as an antenna system used in a base station for wireless communications. Generally speaking, such base stations employ a number of amplifiers, including low noise amplifiers (LNAs). In the case of active antenna systems, the LNAs are mounted on the tower adjacent to the radiating elements or antennas. On the signal receive side of such an installation, a low noise amplifier is typically employed at the tower top for improving the signal-to-noise ratio of the received signal and providing an amplified signal to other base station equipment, which may also be tower mounted, but which is more typically mounted in a base station enclosure at the base of the tower. Failure of tower-mounted equipment presents certain maintenance issues. Since access to the tower mounted equipment requires a worker to actually climb the tower, it is considered desirable to arrange for various fail-safe devices or modes of operation of the tower mounted equipment, including the LNAs.
One such fail-safe mode of operation for a tower-mounted LNA is an automatic bypass capability for bypassing the low noise amplifier (LNA) when a failure mode of the amplifier is detected. Such a failure mode in the LNA results in a significant degradation of the signal output by the LNA. A failure may occur, for example, due to the loss of DC bias current supplied to the tower mounted equipment, which might occur for a number of reasons. In cases of a defined failure mode of the LNA, the LNA may be automatically or otherwise bypassed, feeding the received signals directly from the antenna to the base station equipment, typically at the enclosure at the base of the tower. While such a bypass may degrade the signal-to-noise ratio somewhat in the absence of the amplifier, it is usually considered preferable to the signal degradation, which takes place when the amplifier is in a failure mode. Therefore, it is desirable to provide a bypass arrangement, to bypass an LNA, which is in a predefined failure mode from the absence of a DC control bias.
Currently, relatively expensive and cumbersome RF or microwave-compliant relay circuits have been used as one way to accomplish this bypass function. The switching devices or relays must meet stringent requirements to assure signal quality, including a relatively low insertion loss and relatively high isolation. In addition, a relay system is subject to the usual shortcomings of mechanical switching systems, including relatively low speed, high expense, relatively large size and susceptibility to various mechanical failure modes.
Therefore, it is desirable to bypass a failed LNA of a system in order for the system to continue functioning properly. However, it is also desirable that the bypass function be handled inexpensively and with low loss. A robust, solid-state construction is also desirable. Additionally, the design should function in the bypass mode with all DC input power removed to the mechanism.